Project Summary/Abstract: Innate lymphoid cells (ILCs) represent an emerging family of cell types that express signature transcription factors, including T-bet+ Eomes+ natural killer (NK) cells, T-bet+ Eomes? type 1 ILCs (ILC1), Gata3+ type 2 ILCs (ILC2), and RORgt+ type 3 ILCs (ILC3). ILCs are abundantly present in mucosal tissues (e.g., lung and gut) at the interface of host-environment interactions. ILC1, ILC2, and ILC3 can readily respond to external stimuli (microbes or dietary components) and produce effector cytokines that mirror their adaptive immune counterparts, Th1, Th2, and Th17/22 cells, respectively. Molecular mechanisms underlying the development and function of ILCs are poorly understood. The aryl hydrocarbon receptor (Ahr) is a ligand-dependent transcription factor, best known to mediate the effects of xenobiotic environmental toxins and endogenous microbial and dietary metabolites. We and others have shown that Ahr is required for ILC3 maintenance and function. Our preliminary data revealed tissue-specific expression of Ahr in ILCs and differential regulation of ILC differentiation and function by Ahr in the gut. We hypothesize that Ahr regulates the ILC2-ILC3 balance in the gut, thus impacting intestinal inflammation and immunity. Specifically, we will investigate 1) the regulation of Ahr expression in ILCs, 2) the molecular mechanism(s) by which Ahr regulates the ILC2-ILC3 balance in the gut, and 3) the functional role of Ahr in regulation of ILCs in DSS-induced gut injury. These experiments will offer an opportunity to elucidate environmental impacts on gut inflammation and immunity via the Ahr-mediated pathway. Our study will provide novel cellular and molecular insights into the development and function of ILCs regulated by Ahr in a tissue- and cell-specific manner. Since Ahr is a ligand-dependent transcription factor and its activity can be regulated by small molecules, modulating Ahr expression and/or function in ILCs may represent a new paradigm for human disease treatment or prevention (e.g., inflammatory bowel disease, enteric infections, and allergic diseases).